This is a request for a research grant in support of studies of the role of cell-mediated immunity in chickens infected with Marek's disease virus. The objective of this research is to provide in a unique tumor model system further evidence that the cell-mediated immune response has an important function in resistance of the host to neoplastic disease. The investigations will focus on the identification of virus-specific antigens important for cell-mediated reactions, the characterization of the cell-mediated responses including possible interactions with humoral antibodies, the modifications of the pathogenesis in Marek's disease caused by cell-mediated immune responses, and on the possibility of enhancement of immune responses beneficial to the host.